Fig. 6.—Joining two pieces of tubing end to end—first method.
As the tube softens and tends to shrink, the two ends are pressed together a little and the walls allowed to thicken slightly, as in c. It is then quickly removed from the flame and gently blown as indicated in d, continuing the rotation of the tube during the blowing, and at the same time pressing the ends of the tube together a little so as to make a short thick-walled bulb. The joint is then returned to the flame and reheated, rotating as before, shrinking to about the shape of e. When this stage is reached, the glass should be very hot and fluid, and the mass of hot glass thick enough to remain at its working temperature for about five seconds after removal from the flame. The glass is now reblown as indicated in f, to form a bulb having walls of practically the same thickness as the original tube. As soon as the bulb is blown, the tube is removed from the mouth, held horizontally in front of the worker, and gently drawn out to form one continuous tube, as indicated in g. During both the blowing and drawing of this bulb the rotation must be continued, and both blowing and drawing must be carefully regulated so that the resulting tube may have the same internal and external diameter at the joint as elsewhere.
Discussion.—In making the original joint, (a, Fig. 6), care should be taken that the lump formed is as small as possible so that it may be entirely removed during the subsequent operations. For this reason, only the very tip ends of the two pieces of tubing are held in the flame, and the softening should not extend more than 1⁄16 inch down the tube. As soon as the ends are sufficiently soft to stick together, they are made to do so. The first drawing of the tube (b) should take place immediately, and reduce the lump as much as possible without making the adjacent walls of the tube thin. The whole purpose of the rest of the manipulation is to absorb or "iron out" the lump at the joint. For this reason, care is taken that this lump is always in the center of the flame while the joint is being heated, and a small flame is used so that little of the main tube may be softened. During the first shrinking of the joint (c) the walls next the lump, being thinner than it is, reach the softening temperature first and are thickened by the slight pushing together of the ends, so that they taper from the lump to the unchanged wall. Upon blowing this joint, these thickened walls blow out with the lump, but as they are thinnest next the unchanged tube, they stiffen there first. Then as the thicker parts are still hot, these blow out more, and with the lump make a more or less uniform wall. By this first operation most of the lump will have been removed, provided it was not too large at first, and the tube was hot enough when it was blown. Beginners almost invariably have the glass too cool here, and find difficulty in blowing out a satisfactory bulb. Under such circumstances the lump will be scarcely affected by the operation.
During the shrinking of this bulb, the thinner parts of course are the first to reach the softening point, and thus contract more than the thick parts, so that practically all of the lump can be absorbed, and a uniformly thickened part of the tube left as in e. When this is just accomplished, the second bulb must be blown during one or two seconds, and the tube then drawn out as described, so as to change the bulb to a tube. The drawing must proceed with care: portions nearest the unchanged tubes are the first to reach the proper diameter, and must be given time to just set at that point before the center of the bulb is finally drawn into shape. The drawing is perhaps best done intermittently in a series of quick pulls, each drawing the tube perhaps 1⁄16 inch, and each taking place as the thumbs and first fingers grasp the tube for a new turn in the rotation. If the tube is not rotated during the blowing, the bulbs will be lop-sided and it will be impossible to get a joint of uniform wall-thickness; if rotation is omitted during the drawing, the tube will almost invariably be quite crooked.
If the lump still shows distinctly after the operations described, the cross-section of the tube will be as in h, and the tube will be likely to break if ever reheated at this point after it becomes cold. The operations d, e, f, and g may be repeated upon it, and it may be possible to get it to come out all right.
Care must be taken not to blow the bulbs d and f too thin as they then become very difficult to handle, and the joint is usually spoiled. The wall-thickness of these bulbs must never be much less than that of the original tube. If the joint as completed has thinner walls than the rest of the tube, it will be more easily broken. It should be remembered that the length of the finished tube must be exactly the same as that of the original piece, if the walls of the joint are to be of their original thickness. Therefore the pushing together during the two operations c and d must shorten the tube just as much as the final drawing (f to g) lengthens it.
The interval between the removal of the work from the flame and the beginning of the blowing must be made as short as possible, or else the portions next the main parts of the tube will set before they can be blown out, and cause irregular shrunken areas.